In order to be able for example to establish the heating efficiency or, as the case may be, detect the failure of a heating element, it is necessary to monitor the electrical heating current which flows through the heating elements. This is important in particular during the melting of plastic e.g. in the hot flow channels (hot runners) in the injection tool (mold) in the case of a plastic injection molding machine, since there a small temperature deviation of the plastic melted by means of the heating elements affects the material properties of the plastic melt. This can have a negative impact for example on an injection operation in the case of a plastic injection molding machine for manufacturing plastic parts. In a plastic injection molding machine, plastic granulate is melted for example both in a preplastification unit and in a downstream plastification unit. In an injection tool there can be hot runners in which the temperature of the plastic melt must be maintained. Plastification units and hot runners of said type have heating elements which are fed e.g. by a zero point switching solid-state relay (current relay), with the temperature being controllable by regulating the electric current conducted through the heating elements. Other areas of application for electrical heating apparatuses which have heating elements are, for example, air conditioning systems for clean or ultraclean currents, or heating systems that are used for chemical processes, in which case a reaction chamber for a chemical process, for example, can also be operated in a temperature-controlled manner by means of an electrical heating system.
If the electrical heating system, which is to say the electrical heating apparatus, is used in particular in an area which requires precise regulation of temperature, then it is necessary to monitor the heating efficiency of the electrical heating system accurately at all times. Monitoring of this kind should of course be realizable with little overhead, as well as lead to reliable results. Furthermore measurements in sensitive zones should be carried out quickly and without disrupting the control operation or the control quality in respect of the regulation of temperature. This relates in particular to hot runners.
A known approach to a solution is the use of a summation current transformer for measuring the root-mean-square value of an alternating current. In the case of summation current regulation the summation current of the electrical heating system, in other words the summation current of the different electrical heating elements, is measured and evaluated. This approach has the advantage of low technical overhead. A disadvantage with using a summation current controller is that during the regulation of the temperature all the heating elements are in operation and consequently only the operation of the totality of heating elements can be checked. Checking the heating elements individually is not possible. If it is desired to carry out an individual check, then individual heating elements must always be switched on selectively, while all other runners have to be switched off. However, this causes a disruption to the regulating operation, since during this time the material requiring to be heated can disadvantageously cool down. This problem is also exacerbated thereby when fast-reacting controlled systems are present and when temperature-sensitive material is processed.
Fast and reliable monitoring of the heating current is possible in the case of individual current measuring. This is a reliable monitoring method that does not cause a disruption to a temperature regulating operation. A disadvantage therein, however, is the high overhead that is necessary for this, since every heating element of the electrical heating system requires a sensor such as e.g. a current transformer.